Is hair dye from the supermarket safe?

You have to pay more for better, more professional products. Grocery store selections are more of a inexpensive "quick fix."

Salons may use other, harsher chemicals depending on what you want your hair to look like, but it probably will give you better results that only need "touch-ups" from time-to-time and therefore, in the long run, may be more healthy for your hair. Also, salons can treat or protect damaged hair. Using store dye products, no one but you sees the "big picture" of you hair's overall health.

Also, going darker is better than going lighter regardless because you have to strip the color from the hair which is very damaging.